Here's the short story. I've excavated my pond and dug too deeply on one side (the slope of the yard tricked me when excavating). I've shoveled a bunch of dirt back in, compacted it all with a jumping jack compactor, dug shallow trenches around the outside (tossing this soil into the center part), then tamped it down again. I'm still a few inches (in some places, could be 5") low.
I'll be pouring a 6" slab with #4 rebar on 10" centers. The trenches will make the footers under the walls 10-12" with its own rebar.
Here's my problem - or maybe its not a problem. Parts of the ground are soft. There are a couple of areas where the jumping jack compactor wanted to dig down into the soil. In the picture, you can see some tracks in the clay that were left behind. In some places with the compactor, it felt like I was on plywood that had some give.
Is this a problem? Can I just pour the concrete slab over this and rely on the rebar and thickness of the slab, as well as the distributed nature of the load (nothing compared to cars on a driveway, for instance)? From what I've read, gravel is typically placed under a patio or driveway slab to handle ground water, and to reduce frost heaving. This thing is 3' below my frost line, so that's not an issue. But I'm wondering if gravel is necessary to stabalize the soil before pouring?
Having dug this thing out, and then shoveled a bunch back in, I'm not about to dig it out again to put in a sizeable gravel layer everywhere. But, as I see it, I have three options on how to deal with the soft clay and the variable gap that exists now between where the surface is and where it needs to be raised to pour a 6" slab. I could:
- shovel more dirt and rent the compactor again to raise the surface uniformly. Will still be soft clay
- Leave it as is, and just have thicker concrete in some places.
- Buy a bunch of gravel and use the gravel to even out the surface to the appropriate level. Some parts might not have much gravel, and others might have up to 5" in spots.
I'm guessing that the gravel might help reduce the softness of the clay? There shouldn't be any water issues once this thing is built, as I'll have a french drain (possibly two - one near the surface where water flows and a deeper back up). It's really a question of how solid the dirt has to be before pouring concrete, whether soft clay (even if compacted) is suitable, and if gravel is required or helps.
In this picture, the scale can throw you. The two long legs of the L-shaped pond are about 23' - 25' each. The short wall that is most obvious in the picture (upper left) is over 13'.